Rental income from non-EU state

Does it need to be declared if under 40.000kr?

- Background - I moved to Sweden in 2012 from my native (non-EU) country and have been resident in Sweden uninterruptedly since then. I hold dual citizenships, Sweden and my native country. Before moving to Sweden, I bought properties in my native (non-EU) country. I have received rental income from one of these properties and paid rental income tax in the non-EU country. The rental income has never reached 40.000 kr per year. There is a treaty for the avoidance of double taxation between Sweden and my native (non-EU) country but I can't understand its content fully.

Question: Is it necessary to declare rental income in the case it does not reach 40.000 kr during the year (despite not having to pay any tax)? If yes, what do you suggest that I should do? There is no taxable income for Sweden but I also did not know if I should have declared it.